In a market of image forming apparatus such as laser printer and digital copier, an increased demand is voiced in many features of image forming apparatus such as enhanced image quality, enhanced image-forming speed, further miniaturization of apparatus, and enhanced energy saving, for example.
In view of such trend, an optical writing unit has been employing optical parts having a higher quality and higher performance. Such optical part may include a light source, which irradiates a light beam to an image-carrying member, for example.
In some image forming apparatuses, an optical scanning unit having a light-beam emitting aperture may be disposed under the photosensitive unit.
In such image forming apparatus, when toners spill over or scatter from a photosensitive unit, developing unit, and cleaning unit, such spilled toners may intrude into the optical scanning unit through the light-beam emitting aperture.
Accordingly, the light-beam emitting aperture may be covered with a transparent dustproof member such as glass plate to prevent an intrusion of foreign materials (e.g., toner) into the optical scanning unit through the light-beam emitting aperture.
Such configuration may prevent an intrusion of foreign materials into the optical scanning unit, but foreign materials (e.g., toner) may adhere on the dustproof glass, which may lead to a degradation of image quality.
In related image forming apparatus, an optical scanning unit may include a shutter over a transparent member (e.g., dustproof glass) so that the transparent member may not be contaminated with foreign materials (e.g., toner).
Such shutter may block toner adhesion to the transparent member (e.g., dustproof glass), which may be effective for preventing contamination of dustproof glass.
In one related art, such shutter may be in an opened-condition before starting a developing process for a latent image on the photosensitive unit, and the shutter may be closed just before a completion of the developing process on the photosensitive unit.
In another related art, such shutter may be set to an opened-condition when a given time has lapsed after a synchronous-detecting marking on a photosensitive unit is detected, and the shutter may becomes into a closed-condition when a given time has lapsed after a writing operation in a sub-scanning direction has completed.
However, if the shutter is closed before completing the developing process on the photosensitive member (i.e., the shutter is closed at a wrong timing), a latent image may not be correctly developed on the photosensitive member.
Furthermore, if the shutter becomes into a closed-condition when a given time has lapsed after a writing operation in a sub-scanning direction has completed, the shutter may be in an opened-condition for a time period between a completion of writing operation and a complete closing of the shutter.
In such cases, foreign materials (e.g., toner), dropped by unexpected external shock, may contaminate optical parts related to light beam transmission of the optical scanning unit such as dustproof glass via the above-mentioned still-opened shutter.